[PATCH v8 01/10] landlock: Add landlock_walk_path_up() helper

[Justin Suess <utilityemal77@gmail.com>]

In preparation for centralizing path-walk logic, add
landlock_walk_path_up(), which moves @path one step toward the VFS
root.  Its return value indicates whether the new position is an
internal mount point, the real root, or neither (i.e. the caller
should continue walking).

No functional change intended.

Cc: Tingmao Wang <m@maowtm.org>
Signed-off-by: Justin Suess <utilityemal77@gmail.com>
---

Notes:
    v7..v8 changes:
    
      * Reworded commit message; no code changes.

 security/landlock/fs.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index 3b71f569a8f9..8e75583c3ca7 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -320,6 +320,38 @@ static struct landlock_object *get_inode_object(struct inode *const inode)
 	LANDLOCK_ACCESS_FS_RESOLVE_UNIX)
 /* clang-format on */
 
+/**
+ * enum landlock_walk_result - Result codes for landlock_walk_path_up()
+ * @LANDLOCK_WALK_CONTINUE: Path is now neither the real root nor an internal mount point.
+ * @LANDLOCK_WALK_STOP_REAL_ROOT: Path has reached the real VFS root.
+ * @LANDLOCK_WALK_INTERNAL: Path has reached an internal mount point.
+ */
+enum landlock_walk_result {
+	LANDLOCK_WALK_CONTINUE,
+	LANDLOCK_WALK_STOP_REAL_ROOT,
+	LANDLOCK_WALK_INTERNAL,
+};
+
+static enum landlock_walk_result landlock_walk_path_up(struct path *const path)
+{
+	struct dentry *old;
+
+	while (path->dentry == path->mnt->mnt_root) {
+		if (!follow_up(path))
+			return LANDLOCK_WALK_STOP_REAL_ROOT;
+	}
+	old = path->dentry;
+	if (unlikely(IS_ROOT(old))) {
+		if (likely(path->mnt->mnt_flags & MNT_INTERNAL))
+			return LANDLOCK_WALK_INTERNAL;
+		path->dentry = dget(path->mnt->mnt_root);
+	} else {
+		path->dentry = dget_parent(old);
+	}
+	dput(old);
+	return LANDLOCK_WALK_CONTINUE;
+}
+
 /*
  * @path: Should have been checked by get_path_from_fd().
  */
-- 
2.53.0